1. Technical Field
The present invention relates to electronic telephone instruments and more particularly to an electronic telephone instrument that is capable of dialing (address signaling) in either a pulse mode or in a tone mode.
2. Background Art
Most of the first generation of electronic telephones consisted of either a pulse dial integrated circuit and a voice network integrated circuit plus a number of external discrete components or in the alternative a DTMF tone dialer integrated circuit and a voice network integrated circuit. We will hereinafter refer to a pulse dialer integrated circuit as a "pulse dialer" and the DTMF related version as a "tone dialer". Examples of this type of telephone where the GTE Flip-Phone II.RTM. which employ a pulse dialer, as manufactured by GTE Communication Systems Corporation, and similar telephones manufactured by a number of other manufacturers. The next generation of telephone added memory capability wherein as many as ten complete telephone numbers (station addresses) or more can be stored in the pulse dialer for future recall and dialing out. An example of this type of telephone was the GTE Flip-phone.RTM. III pulse dialer also manufactured by GTE Communication Systems Corporation.
An intermediate or third generation was the combination pulse dialer and tone dialer instrument that included two integrated circuits for dialing, a pulse dialer and a tone dialer and was switchable between the two modes. An example of this type of telephone is the GTE Communication Systems Corporation Linear II.RTM. telephone which employs a Mostek 5175 pulse dialer and control device and the Mostek 5380 tone dialer. While this instrument utilize a coil type voice network it could have employed an integrated circuit for utilization in the voice mode.
The fourth generation of electronic telephones appears to be a switchable pulse-tone instrument with ten or twenty numbers of storage usually of sixteen to twenty-two digits each. This version will be of lower price than the third generation and will become standard as to features and basic configuration of the dialer integrated circuit. The latter will have a certain standard input and output function such as pulse output, tone output, mute output, mode input and hookswitch sensed input to name the basic functions. The keypad interface is usually twelve to sixteen buttons arranged in an XY matrix and directly connected in a four row and three or four column arrangement to the dialer integrated circuit.
A number of problems not solved in the prior art and addressed by this invention which are associated with switchable instruments include the following: (1) correct DC current supervision in both tone and pulse mode; (2) amplification of DTMF signals; (3) proper impedance attained for both voice and tone modes (source impedance and return loss); (4) method of powering the dialer integrated circuit in the pulse mode versus operation in the tone mode; (5) eliminating a second mechanical hookswitch by means of an electronic hookswitch sense circuit.
The electronic voice network (EVN) integrated circuit is a cost saving feature replacing the old style induction coil network. Certain characteristics of a typical electronic voice network are also important to the design of the switchable telephone instruments. These include: (1)necessity for correct circuit impedance (elimination of AC loading by the dial network; (2) a minimum of voltage drops to maintain operation of the circuit at very low loop voltages; (3) the capability of high receive and transmit levels under long loop conditions with low side tone condition; (4) the DTMF amplifiers incorporated must allow for proper DC current supervision source impedance and muting in the tone mode.